The delayed coking process is an established petroleum refinery process which is used on heavy low value residuum feedstock to obtain lower boiling cracked products. Delayed coking can be considered a high severity thermal cracking process. The delayed coking process may be used on residuum feedstocks containing nonvolatile asphaltic materials which are not suitable for catalytic cracking operations because of their propensity for catalyst fouling or for catalyst deactivation by their content of asphaltenes or metals.
Coking of residuum feedstocks from heavy, sour (high sulfur) crude oil is carried out primarily as a means of disposing of low value residuum feedstocks by converting part of the resids to more valuable liquid and gas products. The resulting coke is usually treated as a low value by-product.
The use of heavy crude oils having higher metal and sulfur contents is increasing in many refineries, and delayed coking operations are of increasing importance to refiners. The increasing concern for minimizing air pollution is of further incentive for treating residuum feedstocks in a delayed coker, as the coker produces gases and liquids having sulfur in a form that can be relatively easily removed.
In the delayed coking process, fresh feedstock is introduced into the lower part of a fractionation column and the fractionator bottoms including heavy recycle material (termed flash zone gas oil or natural recycle) and fresh feedstock are heated to a coking temperature in a coker furnace. The hot feedstock then goes to coke drums maintained at coking conditions of temperature and pressure where the feedstock decomposes or cracks to form coke and volatile components. The volatile components are recovered as coke drum vapor and returned to the fractionation column. The heaviest components of the coke drum vapors are condensed by one of several methods, including direct or indirect heat exchange. Typically, heavy coker gas oil from the fractionation column is cooled by heat exchange with fresh feedstock and then returned to the fractionation column where it is sprayed into the fractionator flash zone to contact incoming vapors and condense the heavier components thereof.
The recycled material (termed flash zone gas oil or natural recycle) will be further cracked to lower boiling products which have greater utility even though the yield of coke (“coke make”) is increased by this recycling. The metals and asphaltene content of the heavy coker gas oil product is reduced as the amount of natural recycle is increased.
Present trends in the petroleum refining industry are making it more and more desirable to utilize heavy coker gas oil as hydrocracker feedstock without substantially increasing the natural recycle rate. When heavy coker gas oil is being prepared for hydrocracker feedstock, the amount of natural recycle must be increased (which requires an increase in the amount of flash zone spray material) and downgrades the overall coker yields. Existing delayed coking technology includes increased capital costs due to the addition of a heavy heavy gas oil pump-around system that minimizes the increase in natural recycle to produce the required quality of heavy coker gas oil, or uses shed decks in the flash zone which are prone to coking and plugging unless relatively high natural recycle rates are employed.
Therefore, a need exists for an efficient method of producing heavy coker gas oil of sufficient quality to be used as feedstock for a hydrocracker.